Follow introduction of ColumnDisplayType enum in data explorer comm

[wesm]

Part of posit-dev/positron#2617, adds new top-level enum and renames things accordingly. The summary stats structs aren't used and can be disregarded here for now.

[wesm]

@jmcphers do you know what this test failure is caused by?

test test_kernel ... FAILED

failures:

---- test_kernel stdout ----
thread 'test_kernel' panicked at crates/amalthea/tests/client.rs:437:13:
assertion failed: !comms.contains_key(comm_id)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test_kernel

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.51s

error: test failed, to rerun pass `-p amalthea --test client`

[wesm]

Re-running the build just worked -- is that failure a known flake? If not should create an issue about it. cc also @lionel-